Electrolytic formation of films of fe2o3



United States Patent 3,466,234 ELECTROLYTIC FORMATION OF FILMS OF Fe OMorris Cohen and Peter B. Sewell, Ottawa, Ontario,

Canada, assignors to Canadian Patents and Development Limited, Ottawa,Ontario, Canada, a corporation of Canada No Drawing. Filed June 20,1966, Ser. No. 558,580 Int. Cl. C23b 9/00, 11/00 US. Cl. 204-56 6 ClaimsABSTRACT OF THE DISCLOSURE There is provided a process for depositingferric oxide films from an electrolytic bath containing ferrous iron insolution onto an anodic conductor. The conductors in the electrolyticbath are controlled as to the potentiostat voltage and current densitywhereby the ferrous iron is electrolytically oxidized to ferric oxideand the ferric oxide is deposited as a thin film on the anode. The filmsrange in thickness from A. to about 10,000 A.

The present invention relates to the preparation offerric-oxide-containing materials and particularly to the formation anddeposition of relatively thin films thereof on anodes of various types.The invention also contemplates the transforming of the Fe O -typematerial into 'y-Fe O as well as Fe O Ferric oxide is used widely in thefields of metallurgy, gas purification, pigments, dyeing, andelectronics, being employed in the latter instance in the manufacture ofrecording tapes and in television, radio and radar. It is also known toform good protective coatings but judging from the prior art theapplying and aflixing of ferric oxide coatings has been difiicult to saynothing of the task of preparing the coating mixtures which must ofcourse have good adhering qualities and must be compatible with the basematerial to which they are applied. Where coatings comprising a range ofthicknesses are required it has been diflicult if not impossible in thepast to obtain them.

Applicants have now discovered a method whereby ferric oxide coatingscovering a wide range of thicknesses may be formed by electrochemicaldeposition from solution.

Applicants have found that fine grained compact Fe 0 -type films can beformed by making a conductor anodic under potentiostatic control in asolution containing dissolved ferrous ion. Films of this type may beused to enhance the protectiveness of the ordinarily formed oxide filmon iron, to form colored films on iron or other metals by controllingthe thickness to give various interference colors and to form thinmagnetic or semiconductor films.

The films can be formed on iron, other metallic conductors such asplatinum, zinc and silver. For nonconductors such as plastic tape itwould be necessary to apply a thin film of a conducting material such asiron or other metal by evaporation, sputtering, etc. The properties ofthe oxide can be modified for example by further anodic oxidation in theabsence of Fe++ to produce a cation-deficient outer layer, or if desiredby incorporation of other ions during the anodic electrodepositionprocess, such as other oxidizable cations or complexforming anions.

In carrying out the process of the invention anode and cathode areimmersed in an electrolytic bath containing ferrous (Fe++) iron insolution. A potentiostat is placed in the circuit and a referenceelectrode calomel half cell connected to the anode. The anode asindicated above Patented Sept. 9, 1969 ICC may be any conductingmaterial, for example any metal, or any nonconducting material such aswood, paper, cloth, polymeric material, etc., coated with a conductor.The kinetics of deposition do not vary regardless of the nature of theanodic material, but can be varied by changing the nature of thesolution.

Concerning the film of Fe O it is possible to control the thicknessquite closely by adjusting the concentration of ferrous ion and the timeof deposition, and it is most satisfactory to work under controlledpotential conditions. The deposition takes place at essentially currentefficiency and applicants have deposited films ranging from about 10 A.in thickness to several thousands of A. thickness. The thicker films forexample, up to 10,000 A., contain some water which can be driven off byheating under a vacuum. The: conditions of heating to form Fe O or 'y-FcO are well known. Other ions may also be incorporated into the film bycontrolling the composition of the electrolyte.

Very thin films in the range of about 10 A. to about 100 A. are ofparticular interest in the field of solid state devices. It has beenfound most satisfactory to use dilute solutions of ferrous ion, forexample, 0.5 1O- molar, when depositing films in the very low thicknessregion.

Films are distributed evenly over the entire anode material and are theresult of the ferrous ion (Fe++) in solution being oxidized at the anodeto Fe O or other insoluble ferric compounds. The electrolyte solutionconcentration may range from very dilute to solutions saturated inferrous ion for example 5.0 l0- molar in a borate buffer, pH 8. Wherethe pH is in the neutral range it is desirable to deaerate the solution.

The potential anode to calomel reference may vary widely, applicantshaving deposited films satisfactorily from 200.0 to +8000 millivolts.

The time required for deposition will be determined by the filmthickness desired and also by the current density and the solutionconcentration. The current density may range from 10 to 1000microamperes. per cmfl. The higher the Fe++ concentration the higher isthe possible current.

EXAMPLE Colored Fe O films have been produced by anodic deposition ofoxide film from ferrous ion in a deaerated borate buflfer solution of pH8.0. The color of the film is dependent on the thickness in A. of the FeO layer as shown in the following table, the results having beenobtained using an iron anode at +300 millivolts with the solution being1.0 10* molar in ferrous ion.

Thickness, A. Color Goldish-gray. Light-gray. Gray. Dark-gray.Blueish-gray. Grayish-blue. Grayish-green. Green. Yellowish-green.Pinkish-violet. 1 668. 0 Mauve. 1 1, 900-2, 000 Orange-yellow.

1 On pt.

Results identical with the above may be obtained using platinum as theanode.

If desired transparent coatings may be applied over the colored filmlayer, though the colors are quite stable and durable.

All of the films :set out in the table contain some boron derived fromthe buffer solution.

Buffers such as borate are not essential but may be helpful incontrolling pH.

Generally the films are highly resistant to abrasion and the bonding tothe underlying metal is excellent.

It will be obvious from the foregoing that applicants have devised anextremely versatile, easily manipulated process for applying films of FeO of uniform thickness and distribution to any anodic material.

If desired the Fe O films may be heated under slightly reducingconditions in a controlled atmosphere to form 'Y'F6203.

We claim:

1. The method of depositing fine-grained, compact ferric oxide filmscomprising preparing an electrolytic bath containing ferrous iron insolution, conducting electrodes being immersed in the bath, making oneof the conductors anodic under potentiostatic control, simultaneouslyelectrolytically oxidizing the ferrous iron to ferric oxide anddepositing the ferric oxide in the form of a film on the anode, thevoltage of the potentiostat, anode to calomel reference, being in therange of about 200 to +800 millivolts and the current density beingwithin the range of about 10 to about 1000 microamperes per cm.

2. The method claimed in claim 1 Where the anode is a metallic conductoror a nonconductor having a conducting film surface.

3. The method claimed in claim 1, including the step of heat-treatingthe Fe O films to form 'y-Fe O or Fe O 4. The method claimed in claim 2wherein the nonconducting materials are selected from Wood, cloth, paperand olymeric materials.

5. The process of claim 1 Where the anode is selected from iron andplatinum.

6. A coated article produced by the process of claim 1.

References Cited UNITED STATES PATENTS 3,124,520 3/1964 Juda 204863,294,665 12/1966 Roller 204227 JOHN H. MACK, Primary Examiner R. L.ANDREWS, Assistant Examiner US. Cl. X.R. 20496

